SHANGHAI, Dec 18 (Reuters) - China stocks erased early gains to end lower on Friday, weighed down by fresh signs of tension between the world’s two largest economies. ** Sources told Reuters that the United States is set to add dozens of Chinese companies, including the country’s top chipmaker SMIC , to a trade blacklist on Friday. ** At the close, the Shanghai Composite index was down 0.29% at 3,394.90, while the blue-chip CSI300 index was down 0.35%. ** The smaller Shenzhen index ended down 0.31% and the start-up board ChiNext Composite index was weaker by 0.175%. ** Coal miners, however, outperformed the broader market as some Chinese cities reported increase in electricity shortages. ** Shares of Shanghai Datun Energy Resources Co Ltd , Datong Coal Industry Co Ltd and Anhui Hengyuan Coal Industry and Electricity Power Co Ltd hit their daily upper price limit. ** The state planner was quoted by the official Xinhua news agency as saying that power usage was higher on rapid recovery of industrial production and extreme cold weather in some provinces. It said it would take measures to ensure electricity supply. ** The largest percentage losses in the Shanghai index were Suzhou Longjie Special Fiber Co Ltd, down 9.99%, followed by Chimin Health Management Co Ltd, losing 9.98%, and CRED Holding Co Ltd down by 9.95%. ** Around the region, MSCI’s Asia ex-Japan stock index was firmer by 0.66%, while Japan’s Nikkei index closed down 0.16%. ** So far this year, the Shanghai stock index is up 11.3% and the CSI300 has risen 22.1%, while China’s H-share index listed in Hong Kong is down 6.3%. Shanghai stocks have risen 0.09% this month.